One of the banes of modern society is that dangerous medical conditions without symptoms or risk factors go undetected, often because diagnostic tools and procedures are too expensive or time consuming for routine examinations. One prominent example is coronary disease, such as myocardial ischemia (a lack of blood flow to the heart). Instrumentation currently available in a primary care physician's office often includes electrocardiogram (ECG) equipment. ECG equipment has been used for almost a century to obtain waveforms that represent electric currents flowing within a patient's heart during its beating cycle.
The information available from ECG waveforms is limited mainly by distortions from body tissues. Numerous attempts have been made to derive definitive information from ECG data that indicates coronary disease in patients with no symptoms or risk factors. However, the expense, length of time required, and need for precise placement of probes often severely limit the use of these techniques in mass screenings. Moreover, ECG equipment often cannot detect many cases of coronary disease during a normal or routine physical examination. Owing to the high cost of more reliable tests such as stress echocardiograms and stress myocardial perfusions, patients without symptoms and/or several risk factors often do not receive such tests since they are not covered by insurance.
In contrast, magnetic fields from cardiac currents penetrate body tissues without significant or any distortion. However, these magnetic fields are extremely weak, often about 20 pico-Tesla (pT) peak-to-peak. Still, they can be measured with the aid of an extremely low-level sensor called a Superconducting Quantum Interference Device (SQUID). Various SQUID-type devices have been built, used for clinical testing, and shown to provide far more information about the condition of a patient's heart than ECG. The waveforms produced using a SQUID instrument are called magneto-cardiograms (MCGs) and resemble the waveforms of an ECG, but much more detail is available in an MCG.
Different aspects of MCG waveforms captured using a SQUID instrument have been shown to correlate reliably with various cardiac conditions. For example, T-waves can be detected clearly using a SQUID instrument, and a feature called “T-Wave Alternans” has been shown to be indicative of ischemia of the heart muscle. T-Wave Alternans refers to alternating T-waves shifting up and down in amplitude by about 20%-25%.
Although useful for reliable diagnostics, a SQUID instrument is not practical for routine clinical use. It is quite large and very expensive. It often requires a shielded room to exclude interfering magnetic fields within a spectral range from 0.1 Hz to 100 Hz. In addition, it typically has to be super-cooled so that it is not warmer than 77° K. While some SQUID-type instruments have been developed that do not required the use of a shielded room, these devices still require super-cooling, which often prevents their use in primary clinical settings.